According to an engine temperature controlling technique put into practical use, an engine temperature is controlled to be at a desirable temperature by letting an engine coolant liquid circulate through a heat recovery unit, for example, a radiator. More specifically, a flow volume adjustment valve adjusting a flow volume of an engine coolant liquid according to a position of a valve body is provided to a circulation path in which the engine coolant liquid circulates by passing through a heat recovery unit, and an engine temperature is controlled by adjusting the flow volume adjustment valve (see, for example, Patent Literature 1).